electrical machines 1 lab viva question with answer pdf
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I want viva questions and answers for electrical machines lab 1
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[Q] What happens if the DC supply is applied to the transformer?
Answer: If DC supply is provided to the transformer, the current will not change due to constant supply.
Then mutual induction is not possible and, therefore, the transformer will not work.
The resistance of the primary winding is very small.
For the DC supply, the primary winding inductive reactance is zero.
Therefore, the primary will generate a very high current for the DC supply, therefore, additional heat will be generated and, therefore, the transformer will be damaged.
Therefore, the DC supply must not be applied to the transformer.

[Q] What is the application of the equivalent circuit of a single-phase transformer? [Anna Univ. Dec-2005]
Answer: The equivalent circuit is the electrical model of the transformer. Once the equivalent circuit parameters are obtained, the regulation and efficiency of the transformer can be obtained at any power factor and load conditions without actually charging the transformer.

[Q] What is synchronous speed?
Answer: For synchronous machines there is a fixed relation between the number of poles P, the frequency f and the speed of the machine. The speed of the synchronous machine for the given number of poles and the nominal frequency is called synchronous speed (Ns)
The expression for synchronous speed is
Ns = 120f / P
f = frequency
P = No pole of the machine

[Q] What is meant by torque? or define par?
Answer: A force of twist or torsion around an axis is defined as torque.

[Q] How can we reduce the eddy current loss in the electric machine?
Answer: When using a laminated core construction, stray current loss can be reduced.
The laminated core divides the solid iron core into thin laminations.
The path of the parasitic currents is broken due to the sheets of insulating material between the laminations. Therefore, parasitic currents and the losses generated by them can be minimized.

[Q] In DC generators, the series field winding has low resistance, while the shunt field winding has a high resistance. Why?
Answer: Always the field winding in series is connected in series with the armature.
Therefore, it has to carry the armature current that is directly decided by the load.
Therefore, the field winding of the series that is currently passing is of high level.
The voltage drop in the series field winding is added to the voltage drop in the armature winding while the return emf is decided. This voltage drop must be very small.
Therefore, since the field winding of the current series is high, to keep the voltage drop to a small value, its resistance is very low.
The winding of the bypass field is connected directly through the nominal supply voltage, therefore, to limit the current through it, the resistance is very high.

[Q] Explain the transformer principle? [Anna Univ. May 2010]
Answer: The transformer works on the principle of mutual induction.
Mutual induction:
When two coils are coupled inductively and if the current in one coil is uniformly changed, one emf is induced in the other coil. When the route closes in the second coil, the induced emf can drive a current over it.

[Q] What is the transformer?
Answer: The transformer is a static device by which an electrical power is transformed from one alternating current circuit to another with the desired change of voltage and current without any change in frequency.
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